I found this schem http://www.designnotes.com/CIRCUITS/lowbat.htm and wanted share it here.  I have an old NADY chorus w/a two color LED to show low battery that's pretty nifty, and wondered about doing something similar but using a blinking LED instead. Like any chordless device that blinks at you when it's low.  Anyway, enjoy, and check out the rest of the site, some good stuff.

Maybe some schem posts to Design Notes from DIY stompboxers would help cross-pollinate the whole field...


-onboard

It will be cool to have this indicator in a pedal. Thanks.

Anyone built this yet? I'm puttzing with too many other things... :roll:

Hmm...  Let me know if it works, it seems as though it sort of "meant" for 12v.

-Colin

I took the "9v-max" label at the top of the schem to mean that's it's supply, even though the graph does show up to 12v. Looks to me like the trim pot sets the trip point at 9v or lower, in our case, lower. It is a little unclear if the circuit handles 9v or 12v, my guess would be 9v is within range given the adjustment at R2. I'll definately get around to building it and let 'yuns know. I for one would like a blinking LED when my battery hits 7.5v...gotta find a dying one to set the trip point...

Hi,

what about those "LowCurrent" LEDs
do they work as they can?

in a bunch of parts i got from a friend, are some lowcurrent LEDs
Green and if the Power is under a dedicated point, the light is yello-orange.
there is no battery at the moment in my stock, that is near empty,
so i cant test if the LED will work such as a LowBat. Indicator.
It was not my ambition to use the LED as an indicator, i only needed
green LEDs for my Overdrive...

did anybody used those ones and know, if they work?

bye
Oliver

The silly thing about that circuit is it draws 2 to 4mA for the 741 and a whopping 39mA with R3=100ohm.   The battery checker is sucking more current than the effect!

It doesn't achieve that much more than a simple zener circuit like Boss/Ibanez use - those don't waste any current.

OK, here's a discrete one!  

http://www.siliconchip.com.au/cms/A_30810/article.html

QuoteOK, here's a discrete one!

That one has little or nothing over the boss style.

The boss style unit can be improved slighty using:
- low wattage zeners
- as high a zener voltage as possible (keep in mind too high means the LED will go out but the effect will still operate).
- adding a resistor in parallel with the LED

You can't get much simpler than a series zener!

A low battery indicator that draws more current than the effect is deffinately a no-go. Yet another scenario pointing me in the direction of learning the relevent math/theory, not just firing up the soldering iron, melting away, and marveling at the finished board wondering "Does this one do the magic?" Not to mention the fact that then I could actually contribute.

A low current LED sounds like a great shortcut Oliver, not something I was aware of.

Thanks for the link Paul!  

Hey Boofhead, got a scem??

I'm liking the two color LED approach the more I think about it, anyway, does a low voltage indicator even seem worth using? I mean, it seems some of the mojo out there (depending on the design) actually occurs thanks to a "sagging" battery! Didn't someone draw up a circuit to simulate that?

Man, I'm not logged on half as much as the regulars around here!
Hope this wasn't a dead thread, I'm still interested.

It's too bad that there are't dual color high efficiency LEDs yet. I'm waiting for the high brightness, high efficiency blue-yellow or blue-red dual color LED.

If you used a dual micropower comparator, you could give it two references like a 5.1V zener and a green LED stacked on top of that with a big resistor going to power. Wire one comparitor to output when the voltage is above 5.1V and run that to the green LED. Wire the other comparitor to show when the voltage is below the zener+LED voltage. Wire that to the red LED. Now you have a three color indicator that shows green above ~7.4V, yellow from there down to 5.1V, and red below that.

Take care,
-Peter

Hi.  Peter's idea is a good one.  You can do a similar thing with transistors in place of comparators.  

Based on the fact that transistors switch on with a base voltage above about 0.6V, you can take the battery voltage and divide it down (with 2 resistors) so that 6 or 7V of battery =0.6V at the base.  Simply connect the emitter to ground and the collector to +9V via the led and a current-limiting resistor.  Multiple LEDs can be employed at different voltages (using a series of voltage-dividing resistors).

I just did some calculating, and worked out that for a typical NPN transistor (Hfe between 200 and 400), a voltage divider of 1M and 100k gives about the right switch-on voltage, and a base current low enough that a current-limiting resistor isn't needed.  I think that the led would turn off at about 5V and be fairly bright by 7V (base voltage=.64V, base current=8.4uA, collector/led current =2.5mA)

I must try it sometime....

QuoteHey Boofhead, got a scem??

Just grab any Boss or Ibanez schematic from the web - add the improvements I mentioned if you wish.

What do you actually want out of it?   Do you want the indicator to go off (or change to the other LED) at some precise voltage or do you want one which fades as the battery gets lower.  They are different requirements and require different solutions.  I suppose I'm asking what do you want/expect to battery indicator to indicate?

Quote from: brettI just did some calculating, and worked out that for a typical NPN transistor (Hfe between 200 and 400), a voltage divider of 1M and 100k gives about the right switch-on voltage, and a base current low enough that a current-limiting resistor isn't needed.  I think that the led would turn off at about 5V and be fairly bright by 7V (base voltage=.64V, base current=8.4uA, collector/led current =2.5mA)

I must try it sometime....

I like that idea, is there any chanse you could make a schematic? I'm a little bit dumb :-)
I could then try it and let you know if it works ;-)

Tobias Karlsson in Sweden

Boofhead - I'm liking an LED that comes on when +V is equal to or less than a certain level, 6v ish?

Peter's idea is beyond me ("comparator" is a new word),  thought the traffic light green/yellow/red is good

I get the voltage divider/NPN array. That makes sense to me and I'm sure it works.  

This whole thing is by no means necessary, just an excuse to add another light :wink:

Quote from: onboard("comparator" is a new word)

The opamp in that original schematic is being used to tell if one voltage input is greater than the other. This is called a comparator. You didn't know you already knew . You can make a comparator out of many opamps just by not using any feedback from the output, but comparator chips are made to do just that function so they don't have as many components as opamps and therefore take less power than a standard opamp.

See the LT1441 here: http://www.linear.com/pdf/144012fa.pdf

The LT1441 draws about 3.5uA, or about 1/1000th of an opamp. Your reference resistors will draw much more than that.

Note in the datasheet that they will sink 5mA while sourcing 40mA. Better use LEDs from Vcc through a resistor, and to the outputs if you want more than 5mA drive.

I'll draw up a schematic if you wish.

Take care,
-Peter

I tried the Transistor version of the battery indicator ckt and it seems to be reliable shutting on the LED at 8.75v [where I have it set !!!
  Pretty cool !!!

The transistor solution is much more elegant and simple, but this is how you could do the 3 color with comparators. The green LEDs are just for voltage reference as they're cheaper than zeners.



take care,
-Peter

Thanks Peter S!

But it would be nice to see a scematic of the single tansistor "detector"  

I have a bunch of NPN's so that would be handy. And it should be even less "spacy" at the board  :lol:

Peter M;

Any chanse that you could post a schematic of how you did it?

Tobias Karlsson in Sweden